Hundreds of workers from the SL Garment Processing (Cambodia) Ltd. factory clashed with hundreds of riot police sent to block a march from the factory to the residence of Prime Minister Hun Sen, according to two human rights groups.

Workers from the factory, which makes clothing for Gap, H&M and other international brands, have been protesting for months for better working conditions and pay.

The violence erupted after police sent five officers to negotiate with the protesters, who surrounded and threatened to beat the policemen, said Chan Soveth, senior investigator for the rights group Adhoc.

It was unclear which side started the attacks, in which police fired live bullets and tear gas to rescue their colleagues, and protesters threw rocks and wielded iron bars and wooden sticks, Chan Soveth said.

A woman selling rice near the protest was killed by a police bullet, Chan Soveth and Am Sam Ath of the rights group Licadho said.

The National Police blamed the incident on unidentified "opportunists" who threw rocks and fired slingshots at police and set one of their vehicles on fire. It said in a statement that police took "appropriate measures" to protect themselves and keep the violence from spreading.

It said 27 policemen had been wounded, nine critically, and one woman had been killed.

Six protesters were injured, Chan Soveth said, adding that five Buddhist monks were hurt when police fired tear gas into a Buddhist pagoda where demonstrators had sought refuge. Nine other people were hurt, including youths who jumped into the clashes and bystanders, he said.

"The crackdown conducted by police this morning against workers was very cruel and unacceptable," said Kong Athith, a spokesman for the Coalition of Cambodian Apparel Workers Democratic Union, which organized the protest. "Workers were unarmed. Why did police use live ammunition to crack down on them?"

The garment industry is Cambodia's biggest export earner, employing about 500,000 people in more than 500 garment and shoe factories. In 2012, the Southeast Asian country shipped more $4 billion worth of products to the United States and Europe.

Pay is low and working conditions are usually uncomfortable. In May, the ceiling of a Cambodian footwear factory collapsed, killing two people and injuring seven.

(Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)